This invention relates to the art of display systems and, more particularly, to apparatus for compensating for nonlinear response characteristics thereof.
The invention is particularly useful in compensating for nonlinear response characteristics of a cathode ray tube. However, it is to be appreciated that the invention has broader applications and may, for example, be used to compensate for nonlinear response characteristics of other display devices, such as devices in which the beam is generated by a laser mechanism.
Cathode ray tubes have long been used to provide visual displays. More recently, cathode ray tubes have found increasing use in the art of phototypesetting. Control of these devices has been hampered by various nonlinear response characteristics inherent therein. Pin cushion distortion, for example, arises as a result of the nonlinear relation between the control signal applied to the beam deflection system and the acutal beam deflection produced thereby. It is desirable to provide a specialized circuit for correcting this nonlinearity. Such a correcting circuit would characteristically be interposed between the source of the control signals and the display device. The nonlinear response characteristic of this correction circuit would combine with the nonlinear response of the display device to provide an overall linear relation between the control signal applied to the correction circuit and actual deflection of the beam.
Deflection systems commonly provide deflection of the radiant energy beam in two orthogonal directions whereby the beam can be positioned at any point on a display surface through control of two deflection circuits. Correction of the non-linearity in deflection response of these systems is hindered by the interdependency of the corrections necessary along the axes. In other words, the correction to be applied along either axis is a function of the deflection to be achieved along both axes.
U.S. Pat. Nos. 3,825,796 and 3,512,039 exemplify several previous methods of achieving the necessary correction. One previous method combines a monotonic correction voltage together with the control signal so that the amount of the correction continually increases with increasing control signals. Because of this, the correction signal becomes quite large near the extremities of the display surface. These correction signals have proved to be difficult to generate accurately.
It is essential, particularly in phototypesetting systems, that this correction be as exact as possible. Additionally, it is desirable that the characteristics of the circuit be easily matched to the individual characteristics of a specific display device. The circuit should also be uncomplicated and economical to manufacture.
The present invention accomplishes these goals by providing apparatus which generates a correction signal and combines this signal with an uncompensated control signal to provide a compensated radiant energy beam deflection signal. The compensation signal so produced is functionally related to the control signal in a non-monotonic relation so that the amount of correction remains relatively small throughout the entire display surface. On-axis and off-axis correction signals are independently produced so as to simplify any necessary adjustments thereto. Circuitry is provided for developing correction signals for only a single quadrant of the total display surface, and additional circuitry adjusts and inverts the correction signal so produced so as to provide correction signals which are proper for the actual quadrant in which the control signals lie.